From the journal Environmental Science: Atmospheres Peer review history

27-Sep-2021

Dear Dr Styler:

Manuscript ID: EA-ART-09-2021-000070
TITLE: Ozone uptake by commercial brake pads: assessing the potential indirect air quality impacts of non-exhaust emissions

Thank you for your submission to Environmental Science: Atmospheres, published by the Royal Society of Chemistry. I sent your manuscript to reviewers and I have now received their reports which are copied below.

I have carefully evaluated your manuscript and the reviewers’ reports, and the reports indicate that major revisions are necessary.

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************

Reviewer 1

The paper addresses the timely topic of the contribution of brake wear particle from car brake pad.
Especially in Europe and the United States, there is a growing interest in the impact of non-exhaust particles on the air quality.
There is a lack of information on the behavior of non-exhaust particles in the atmosphere, their emissions, their lifetime in the atmosphere, and their interactions with other materials.
In Europe and the United States, there is a growing interest in the impact of non-exhaust particles on the air quality.
This paper describes the experimental determination of the ability of automotive brake particles to uptake atmospheric ozone. Even though the results based on the variable and limited experiment, this paper gives reliable results and novel concept for the characterization of the brake wear particles.
This paper is well structured and clear.
This paper provides very interesting data and acceptable, but it still needs a considerable revision to be made before publication.


Specific Comments:

--- Abstract "brake wear particles"
The "brake wear particles" used in the experiments in this paper are very different from the "particles generated by the wear of actual automobile brakes".
Aerosol particles originating from brake wear are generated when wear particles generated by complex tribological phenomena occurring at the friction interface are dispersed into the air. The ablative wear mechanism is caused by the abrasive material between the friction interfaces, the generation mechanism is caused by the mechanochemical reaction from the transfer layer generated by the shear force, pressure and heat of solid lubricants, metal additives and resins between the friction interfaces, the nucleation mechanism is caused by the condensation of evaporated gas due to the partial dissolution of the material surface to generate aerosol particles, and the aerosol particle generation mechanism is caused by the chemical reaction between the material surface and the friction interface. Aerosol particles are generated by very complicated phenomena, such as the generation mechanism by mechanochemical reactions from the advection layer generated by shear force, pressure and heat, and the nucleation mechanism by which evaporated gas condenses to produce aerosol particles due to partial dissolution of the material surface.
The "brake wear particles" obtained in this study are, from the viewpoint of an expert, "model particles of milled brake friction materials".
The title and abstract are misleading to the readers and degrade the integrity of the journal. For this reason, the term "brake wear particles" should be changed to "model particles of milled automotive brake friction materials".

--- Abstract
--- Line 43-46:
The last sentence of the article, "brake wear toxicological properties" in the last sentence is not directly related to the findings of this study.
The purpose of this journal is to obtain scientific knowledge referring to "air quality impacts" and "atmospheric reactivity". The "brake-wear-ozone interactions" in this paper have not resulted in any significant or rational findings that would lead to "toxicological properties".
Therefore, the findings from the "brake wear-ozone interactions" should be used to describe what "air quality impacts" and "atmospheric reactivity" findings should be obtained next.
For example, "we suggest that future studies of brake wear-ozone interactions focus on their potential impact on wet deposition and fate." could be considered.

--- Environmental Significance Statement
--- Line 43-46:
Same as Abstract.
The "brake-wear-ozone interactions" in this paper have not resulted in any significant or rational findings that would lead to "toxicological properties".
The last sentence should be a reasonable description of the other "air quality impacts" and "atmospheric reactivity".

--- 2.2 Experimental procedure
--- Line 104-105:
--- mimic the fraction of brake wear
Reference 9 does not prove or show reasonable data that this experimental method can be a "mimic the fraction of brake wear" compared to "actual brake wear particles".
Therefore, it should be changed to "model particles of milled automotive brake friction materials".
Be sincere in stating the facts.

--- 3.4 Atmospheric significance
--- Line 278-281:
In this paper, I strongly recommend "beyond the scope of this study, ozone uptake by brake wear could also potentially influence its toxicological properties", therefore the sentence in Line 278-281 should be deleted.

Since Ca and Ba are also contained in brake friction materials, it would be better if the amount of ozone deposited on the friction materials is reasonably discussed in this journal by referring to the following references, for example.

S Feil , G K Koyanagi, A A Viggiano, D K Bohme: Ozone reactions with alkaline-earth metal cations and dications in the gas phase: room-temperature kinetics and catalysis, J Phys Chem A. 2007;111(51):13397-402

Reviewer 2

This is an interesting manuscript describing the ozone uptake by commercial brake pads.
As such, the manuscript is well written, presented, and easy to follow even for a mechanical engineering researcher as I. Before publication, I recommend some amendment of the manuscript.

1. There are two surfaces in contact in a mechanical brake system as disc brakes. Only one of them are studied in the manuscript. To the best of my understanding, the airborne particulates from mechanical brake systems are often a mix of these components in their chemistry. The chemistry of the particulates are seldom isolated to originate from only one contacting surface. This needs to be discussed in the discussion section of the results.
2. The studied particulates are generated with a grinder mixer. As I see it this is a process far away from the processes that generates airborne particulates from disc brake surfaces in contacts. Are the grinder mixer particulates only from the pad material or from the combination of pad and grinder mixer material? Are the particulates studied thermally stressed in a different way from the ones generated in a mechanical disc brake contact? There is usually a semi volatile part of the emissions from disc brake surfaces. Is this semi volatile part present in the grinder mixer process and is such a volatile composition import for the ozone uptake?
3. Airborne particulates from disc brakes vary typically between 1 nm to 40 µm in size with a mass mode around 2 µm and a number mode around 3 – 20 nm. Please clarify the size distribution of particles from the grinder mixer and discuss how any differences from normal disc brake particulates can affect the presented results?

This text has been copied from the PDF response to reviewers and does not include any figures, images or special characters.

“Ozone uptake by commercial brake pads and brake pad components: assessing the potential indirect air quality impacts of non-exhaust emissions”, by Matchett et al. (EA-ART-09-2021-000070)— response to reviewers

Reviewer 1

1 "The paper addresses the timely topic of the contribution of brake wear particle from car brake pad. Especially in Europe and the United States, there is a growing interest in the impact of non-exhaust particles on the air quality. There is a lack of information on the behavior of non-exhaust particles in the atmosphere, their emissions, their lifetime in the atmosphere, and their interactions with other materials. In Europe and the United States, there is a growing interest in the impact of non-exhaust particles on the air quality. This paper describes the experimental determination of the ability of automotive brake particles to uptake atmospheric ozone. Even though the results based on the variable and limited experiment, this paper gives reliable results and novel concept for the characterization of the brake wear particles. This paper is well structured and clear. This paper provides very interesting data and acceptable, but it still needs a considerable revision to be made before publication."

Thank you!

2 “--- Abstract "brake wear particles"
The "brake wear particles" used in the experiments in this paper are very different from the "particles generated by the wear of actual automobile brakes". Aerosol particles originating from brake wear are generated when wear particles generated by complex tribological phenomena occurring at the friction interface are dispersed into the air. The ablative wear mechanism is caused by the abrasive material between the friction interfaces, the generation mechanism is caused by the mechanochemical reaction from the transfer layer generated by the shear force, pressure and heat of solid lubricants, metal additives and resins between the friction interfaces, the nucleation mechanism is caused by the condensation of evaporated gas due to the partial dissolution of the material surface to generate aerosol particles, and the aerosol particle generation mechanism is caused by the chemical reaction between the material surface and the friction interface. Aerosol particles are generated by very complicated phenomena, such as the generation mechanism by mechanochemical reactions from the advection layer generated by shear force, pressure and heat, and the nucleation mechanism by which evaporated gas condenses to produce aerosol particles due to partial dissolution of the material surface. The "brake wear particles" obtained in this study are, from the viewpoint of an expert, "model particles of milled brake friction materials". The title and abstract are misleading to the readers and degrade the integrity of the journal. For this reason, the term "brake wear particles" should be changed to "model particles of milled automotive brake friction materials".”
We appreciate this thorough discussion of brake wear formation mechanisms. We have included some of this information in our manuscript to strengthen our discussion of the differences between our sample generation procedure and that of real-world brake wear (see our response to Reviewer 1 Comment 5 below).
We completely agree that our particles are not “real-world” brake wear particles; in this context, we carefully considered which terms to use in our manuscript. We used the term “brake pads” when referring to the ground brake pad samples used in our experiments and the term “brake wear” when referring to ‘real-world’ brake wear PM (e.g., when providing background information or contextualizing results). We concede that this differentiation may be confusing to readers, who will encounter these terms in the abstract and introduction text prior to reading the description of our grinding method that we provide in the experimental section. To make this differentiation clearer and remove any ambiguity as to which particles we are referencing, we have made the following amendments to the text:
Table of content entry:
“We determine, for the first time, the reactivity of ground brake pads with ozone and discuss the potential impact of this interaction on urban air quality.
Abstract:
“To address In a first step toward addressing this knowledge gap, we explored the reactivity of ground brake pads (ceramic, semi-metallic, organic) and common brake pad components (phenolic resin, graphite, Fe2O3, Fe3O4, Fe and Cu powders) with ozone, an important urban pollutant.”
“Our γBET values suggest that ozone loss to “real-world” brake material wear may be larger than to organic PM during high-traffic periods.”
Environmental significance statement:
“Here, we investigate the reactivity of ground commercial brake pads and brake pad components with ozone, a model-gas model gas-phase pollutant.”
Introduction:
“Here, using ozone as a model gas-phase pollutant and ground brake pad particles as a simplified proxy for brake wear PM generated under mild conditions proxy for mechanically generated brake wear, we investigate for the first time the atmospheric reactivity of material relevant to vehicle non-exhaust PM.”
Experimental procedure:
“All brake pad experiments were conducted using freshly ground samples, which were produced by grinding commercial brake pads (30 s, 3800 rpm) in a Wig-L-Bug grinder mixer equipped with a stainless steel vial (1.3 cm id, 2.5 cm length; 3.3 cm3 volume) and a single stainless steel ball pestle (0.6 cm diameter). Hereafter, we use the term “brake pads” to designate this material. which we used to mimic the fraction of brake wear mechanically generated by the physical compression of the pad against the rotor.9”
“To better understand the reactivity of brake material over both short and extended ozone exposure timescales, brake wear over its atmospheric residence time, we report both time-dependent (γBET,t; 15, 30, and 60 min ozone exposure) and steady-state (γBET,SS) uptake coefficients for all samples.”
Time-dependent ozone uptake by brake materials:
“As shown in Fig. 4a, γBET values for all brake pads are higher at shorter ozone exposure times; assuming that the time-dependent reactivity profile of emitted brake wear PM qualitatively resembles that of the parent brake material studied here, these results suggest indicating that freshly emitted brake wear will be a more reactive surface for ozone uptake than atmospherically aged brake wear.”
Atmospheric significance:
“in particular, using typical PM2.5 brake wear loadings (1.5 µg m–3)43,44 and γBET,30min, γBET,60min, or γBET,SS values for our brake pads, and assuming that the reactivity of brake wear PM can be approximated by that of parent brake material, we find that ozone loss to brake wear is always < 1% of its loss to the ground (detailed calculations are presented in the ESI). Barring a situation in which ozone reactivity with brake wear PM is orders of magnitude larger than its reactivity with parent brake material, Thus, we conclude that brake wear chemistry alone will have minimal impact on urban ozone mixing ratios.”
“Using estimated PM loadings from a study in Toronto, Canada,43 and assigning γBET values based on published reactivity data for organic PM,25 and using γBET,30min obtained here for ceramic and organic PBR brake pads (and again assuming that parent brake material is a reasonable proxy for atmospheric brake wear PM), we conclude that freshly emitted brake wear has the potential to be can be a more important ozone sink than organic PM (detailed calculations are presented in the ESI).”


3 “--- Abstract--- Line 43-46: The last sentence of the article, "brake wear toxicological properties" in the last sentence is not directly related to the findings of this study. The purpose of this journal is to obtain scientific knowledge referring to "air quality impacts" and "atmospheric reactivity". The "brake-wear-ozone interactions" in this paper have not resulted in any significant or rational findings that would lead to "toxicological properties". Therefore, the findings from the "brake wear-ozone interactions" should be used to describe what "air quality impacts" and "atmospheric reactivity" findings should be obtained next. For example, "we suggest that future studies of brake wear-ozone interactions focus on their potential impact on wet deposition and fate." could be considered.”

We thank the reviewer for their comments (3, 4, 6) related to our discussion of “toxicological properties” and agree that the manuscript would benefit from a broader discussion of the implications of the oxidation of non-exhaust PM. As such, we have amended the environment significance statement and atmospheric significance sections (as described in our responses to Comments 4 and 6, respectively) and added the following text to the abstract:

“As this loss pathway is still small compared to ozone dry deposition, we suggest that future studies of brake wear–ozone interactions focus on their potential to change brake wear properties (e.g., hygroscopicity, toxicity) relevant in an air quality context. Impact on brake wear toxicological properties.

4 “--- Environmental Significance Statement--- Line 43-46:
Same as Abstract. The "brake-wear-ozone interactions" in this paper have not resulted in any significant or rational findings that would lead to "toxicological properties".
The last sentence should be a reasonable description of the other "air quality impacts" and "atmospheric reactivity".

The text of our environmental significance statement now reads as follows:

“However, as the oxidation of brake wear and other non-exhaust PM types could potentially lead to changes in their toxicological properties, further interdisciplinary studies of this emerging class of urban PM are warranted.”

5 --- 2.2 Experimental procedure --- Line 104-105: --- mimic the fraction of brake wear Reference 9 does not prove or show reasonable data that this experimental method can be a "mimic the fraction of brake wear" compared to "actual brake wear particles". Therefore, it should be changed to "model particles of milled automotive brake friction materials". Be sincere in stating the facts.

We thank the reviewer for pointing out our misplacement of Reference 9. We were using this reference only to define mechanically generated brake wear and did not intend for readers to infer that it provided evidence to support the idea that our ground brake pad particles were mimicking mechanically generated brake wear; however, we acknowledge that the placement of the reference was inadvertently misleading and have changed this section of the text accordingly. In particular, we have added clarification text (and moved some of the text from the atmospheric significance section) to more clearly explain why we prepared our particles in this way.
“All brake pad experiments were conducted using freshly ground samples, which were produced by grinding commercial brake pads (30 s, 3800 rpm) in a Wig-L-Bug grinder mixer equipped with a stainless steel vial (1.3 cm id, 2.5 cm length; 3.3 cm3 volume) and a single stainless steel ball pestle (0.6 cm diameter). Hereafter, we use the term “brake pads” to designate this material. which we used to mimic the fraction of brake wear mechanically generated by the physical compression of the pad against the rotor.9 Brake wear can also be formed via thermal and oxidative processes, which can alter the composition of emitted PM with respect to the original brake pad;9 We used ground brake pads rather than brake wear generated from a vehicle or brake dynamometer for two reasons: first, mechanical abrasion is an important process in brake wear generation;9 second, brake dynamometers produce only small amounts of sample, are expensive and time-consuming to run, and introduce multiple variables to the brake wear generation process (e.g., initial speed, deceleration rate), all of which would have limited the range of samples investigated here. Importantly, studies have shown that the composition of emitted brake wear PM differs from that of the parent material; we discuss the potential reactivity impacts of these compositional differences in Section 3.4.”

In response to this comment and two others (Reviewer 1 Comment 2 and Reviewer 2 Comment 3), we also now refrain from directly comparing our ground samples to mechanically generated brake wear, as we recognize that actual brake wear generation processes are much more complicated and interconnected than we may have been leading readers to believe. We have amended the following text in the atmospheric significance section:

“As discussed in the Experimental Procedure section, we used ground brake pads as model particles of brake friction material. However, “real-world” brake wear (i.e., particles generated by the wear of actual automobile brakes) is formed through multiple simultaneous and complex processes, including abrasion of friction interfaces (i.e., between the brake pad and brake disc); mechanochemical reactions occurring due to the shear force, pressure, and heat generated during braking; and the evaporation and subsequent condensation of volatile species.9 Consequently, the composition of brake wear PM can differ from the parent brake material: for example, the organic components may degrade, and the iron may become oxidized during braking.38 Based on our results, however, the effects of these changes on overall sample reactivity are inconclusive. For example, the degradation of organic resins may lead to a decrease in reactivity (Fig. 3b), whereas the oxidation of iron may or may not affect reactivity, as all forms of iron we tested (Fe powder, Fe2O3, and Fe3O4) had similar reactivity (Fig. 3b). Additionally, “real-world” brake wear also includes iron and iron oxide particles from the brake disc (typically cast-iron),16 which, based on our results (Fig. 3b), may contribute to the overall reactivity of this PM class.”







6 --- 3.4 Atmospheric significance --- Line 278-281:
In this paper, I strongly recommend "beyond the scope of this study, ozone uptake by brake wear could also potentially influence its toxicological properties", therefore the sentence in Line 278-281 should be deleted.

As noted in our responses to Comments 3 and 4 from this reviewer, we have expanded our discussion of the potential impacts of oxidation on brake wear properties and at the same time ensured that the focus remains on air quality impacts. The relevant text now reads as follows:

“Ozone uptake by organic PM has been shown to alter a wide variety of PM properties, including hygroscopicity,34,36 viscosity,45,46 and toxicity.47,48
As we show above that the overall effect of brake wear PM on urban ozone mixing ratios is most likely small, we suggest that future research in this area focus on the potential impacts of brake wear–ozone interactions on brake wear properties, which has received little attention to date20.

7 Since Ca and Ba are also contained in brake friction materials, it would be better if the amount of ozone deposited on the friction materials is reasonably discussed in this journal by referring to the following references, for example.

S Feil , G K Koyanagi, A A Viggiano, D K Bohme: Ozone reactions with alkaline-earth metal cations and dications in the gas phase: room-temperature kinetics and catalysis, J Phys Chem A. 2007;111(51):13397-402

Based on our elemental composition analyses, Ca and Ba are present as minor components of our brake pad samples (< 4% for Ca and < 1% for Ba); as we state in Section 3.2 of our manuscript, these (and other) minor components may also contribute to overall brake pad reactivity. However, we specifically limited our more detailed discussion of component contributions to observed reactivity (and potential mechanisms underlying these contributions) to the most abundant brake pad components, which we directly studied using our coated-wall flow tube reactor.

We thank the reviewer for providing the suggested reference; we have not cited it, however, as it focuses on the reactivity of ozone with gas-phase Ca2+ and Ba2+, rather than with the crystalline oxide forms of these elements. One study has reported that the reactivity of ozone with calcite (CaCO3), which is likely the form of Ca present in brake pads, is low. To our knowledge, the reactivity of ozone with baryte (BaSO4), which is the Ba species identified in our brake pad samples, has not been investigated.






Reviewer 2

1 “This is an interesting manuscript describing the ozone uptake by commercial brake pads. As such, the manuscript is well written, presented, and easy to follow even for a mechanical engineering researcher as I. Before publication, I recommend some amendment of the manuscript.”

Thank you!

2 “There are two surfaces in contact in a mechanical brake system as disc brakes. Only one of them are studied in the manuscript. To the best of my understanding, the airborne particulates from mechanical brake systems are often a mix of these components in their chemistry. The chemistry of the particulates are seldom isolated to originate from only one contacting surface. This needs to be discussed in the discussion section of the results.”

Yes, brake wear indeed contains contributions from both brake pads and brake discs, which are in contact during braking. We have amended our discussion of the differences between our ground brake pad particles and “real-world” brake wear as follows:

“As discussed in the Experimental Procedure section, we used ground brake pads as model particles of brake friction material. However, “real-world” brake wear (i.e., particles generated by the wear of actual automobile brakes) is formed through multiple simultaneous and complex processes, including abrasion of friction interfaces (i.e., between the brake pad and brake disc); mechanochemical reactions occurring due to the shear force, pressure, and heat generated during braking; and the evaporation and subsequent condensation of volatile species.9 Consequently, the composition of brake wear PM can differ from the parent brake material: for example, the organic components may degrade, and the iron may become oxidized.38 Based on our results, however, the effects of these changes on overall sample reactivity are inconclusive. For example, the degradation of organic resins may lead to a decrease in reactivity (Fig. 3b), whereas the oxidation of iron may or may not affect reactivity, as all forms of iron we tested (Fe powder, Fe2O3, and Fe3O4) had similar reactivity (Fig. 3b). Additionally, “real-world” brake wear also includes iron and iron oxide particles from the brake disc (typically cast-iron),16 which, based on our results (Fig. 3b), may contribute to the overall reactivity of this PM class.”

3 “The studied particulates are generated with a grinder mixer. As I see it this is a process far away from the processes that generates airborne particulates from disc brake surfaces in contacts. Are the grinder mixer particulates only from the pad material or from the combination of pad and grinder mixer material? Are the particulates studied thermally stressed in a different way from the ones generated in a mechanical disc brake contact? There is usually a semi volatile part of the emissions from disc brake surfaces. Is this semi volatile part present in the grinder mixer process and is such a volatile composition import for the ozone uptake?”

Thank you for your thoughtful questions. As you say, our particle generation process is different than that resulting in release of real brake wear particles, in particular with respect to thermal stress. We purposefully kept our grinding time short (30 s) such that no noticeable increase in the temperature of the vial could be felt after grinding; however, we cannot exclude the possibility that certain local spots in our samples may have reached higher temperatures. Nonetheless, temperatures reached during braking are much higher than the temperatures we believe our samples experienced during the grinding process.

Additionally, during actual braking, PM generation processes (e.g., mechanical contact, thermal/oxidative processes) occur simultaneously, generating complex particles that differ from the original brake pad material. To better capture this complexity in our manuscript, we have added a more thorough discussion of brake wear generation processes to our atmospheric significance section (please see our response to Reviewer 1 Comment 5).

In terms of potential contamination from the grinder vial and/or mixer material, the Wig-L-Bug we employed uses a small stainless steel cylindrical vial (1.3 cm id, 2.5 cm length; 3.3 cm3 volume) with a single stainless steel ball pestle (0.6 cm diameter). We expect that any contamination of samples from the vial/pestle was minimal. This conclusion is supported by the fact that we saw substantially different reactivities for our sample suite (i.e., if the reactivity contribution from vial/pestle material were larger than that from the samples themselves, we would expect the apparent reactivity of samples to be similar). In response to this question, we have added the following text to the manuscript:

“All brake pad experiments were conducted using freshly ground samples, which were produced by grinding commercial brake pads (30 s, 3800 rpm) in a Wig-L-Bug grinder mixer equipped with a stainless steel vial (1.3 cm id, 2.5 cm length; 3.3 cm3 volume) and a single stainless steel ball pestle (0.6 cm diameter).”

As for the formation of semi-volatiles, this can occur during braking via the thermal and oxidative degradation of brake pad organics (e.g., phenolic resin), and thus could theoretically also occur during grinding. According to our thermogravimetric analysis data (Figure S3), our brake pad samples do not begin to degrade until ~250°C; as we did not notice an increase in the overall temperature of our grinding vial over the 30 s grinding period (see above), we do not believe that these processes occurred during the grinding process employed in this study.

We note that we cannot exclude the possibility that local temperatures may have exceeded 250°C during grinding; if this were the case, semi-volatile species may have been formed (and possibly released). Importantly, however, as coated tubes were heated (~20 h at 50–100°C) prior to use, any reactivity conferred by these species was likely minimized by their release to the gas phase.

4 “Airborne particulates from disc brakes vary typically between 1 nm to 40 µm in size with a mass mode around 2 µm and a number mode around 3 – 20 nm. Please clarify the size distribution of particles from the grinder mixer and discuss how any differences from normal disc brake particulates can affect the presented results?”

During the method development stage of this project, we tested multiple brake pad grinding durations; for each, we obtained microscope images of the particles produced (both to ensure that our particle size was reasonable and to verify that it would coat well on the reaction tube). We obtained these images by placing a small amount of ground sample on a microscope slide; as we did not use dispersant (and particles were present largely as aggregates), this strategy allowed us to see if the particles were small enough for tube coating but did not permit us to obtain quantitative size distributions. From a very basic visual inspection of the images obtained, we can say that a) many of the primary particles had diameters between 2–5 μm; and b) the particle assemblage also included particles or aggregates with diameters > 10 μm. Since we used the same procedure to grind all samples, we anticipate that the size distribution for each trial of the same brake pad sample was similar, but that the size distribution may have differed from sample to sample.

The size distribution of the particles studied here is likely quite different than that of ‘real-world’ brake wear PM, as the braking process produces coarse, fine, and ultrafine PM. To account for these differences, as well as potential differences in the post-grinding size distributions of different brake pad types and brake pad components, we normalized all uptake coefficients using the BET specific surface area of each sample type.

Importantly, reproduction of the size distribution of ‘real-world’ brake wear was not one of our goals here, as the ultrafine mode is produced by a fundamentally different mechanism (evaporation and subsequent condensation of brake pad organic components, which requires high temperatures) and consists of particles with composition drastically different than the parent brake pad material. We believe that this compositional difference would be the major driving factor underlying potential reactivity differences between our results and real-world brake wear; we now have addressed this in more detail in the Supplementary information.

16-Feb-2022

Dear Dr Styler:

Manuscript ID: EA-ART-09-2021-000070.R1
TITLE: Ozone uptake by commercial brake pads and brake pad components: assessing the potential indirect air quality impacts of non-exhaust emissions

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Reviewer 2

Thank you for answering all my previous questions